Alignment of variable storage in structures in C Language

In C, the Declaration Order of the variables in the struct varies with the bytes.

For example,

 
Struct {short X; long y; char Z;};
 

The preceding struct occupies 12 bytes.

 
Struct {short X; char y; long Z;} B;
 

This struct occupies 8 bytes.

The reason is that the compiler has alignment problems.

In the first struct a, X occupies 2 bytes, but y occupies 4 bytes. It cannot be directly supplemented after the space in X. In order to align y with X, the second byte after the space occupied by X is wasted and not used. Z occupies 4 bytes to maintain alignment. A total of 12 bytes.

In the second struct B, X occupies 2 bytes, and y occupies 1 byte. According to the alignment principle, the next byte of Y occupies is wasted. The remaining variable z occupies 4 bytes. A total of 8 bytes.

The following figure shows how

Shows the space occupied by the first struct a. Black represents the bytes that store the data, and red represents the bytes that do not store the data.

Shows the space occupied by the second struct B,Black indicates the bytes of the stored data, and red indicates the bytes of the data that are not stored.